Add new GUC createrole_self_grant.

Can be set to the empty string, or to either or both of "set" or
"inherit". If set to a non-empty value, a non-superuser who creates
a role (necessarily by relying up the CREATEROLE privilege) will
grant that role back to themselves with the specified options.

This isn't a security feature, because the grant that this feature
triggers can also be performed explicitly. Instead, it's a user experience
feature. A superuser would necessarily inherit the privileges of any
created role and be able to access all such roles via SET ROLE;
with this patch, you can configure createrole_self_grant = 'set, inherit'
to provide a similar experience for a user who has CREATEROLE but not
SUPERUSER.

Discussion: https://postgr.es/m/CA+TgmobN59ct+Emmz6ig1Nua2Q-_o=r6DSD98KfU53kctq_kQw@mail.gmail.com

src/backend/utils/misc/postgresql.conf.sample

@@ -703,6 +703,7 @@
 #xmlbinary = 'base64'
 #xmloption = 'content'
 #gin_pending_list_limit = 4MB
+#createrole_self_grant = ''		# set and/or inherit
 
 # - Locale and Formatting -